Honestly, I think you're asking the wrong question entirely. If Bob's work and knowledge are valued in the company at all, you need to look at the bigger picture and not just in the context of your team.

Bob was promised a transfer, the company failed to deliver. Perhaps his patience is exhausted and he's looking to move to another company that follows through on its commitments, values, and respects its employees.

Outright informing a superior or team lead that Bob is looking for another job put's him in a negative light, ultimately placing the blame on him and opening him up to immediate termination. As others have pointed out it is questionable behavior on your part

Approaching a superior or team lead and saying that we're may lose Bob, an important part of the team, because the company didn't follow through on its promise of a transfer puts things in an entirely different light. It also opens up opportunities for the company to address the underlying issues and allows the company to address them if they chose to.

So what is your goal here?

Honestly, I think you're asking the wrong question entirely. If Bob's work and knowledge are valued in the company at all, you need to look at the bigger picture and not just in the context of your team.

Bob was promised a transfer, the company failed to deliver. Perhaps his patience is exhausted and he's looking to move to another company that follows through on its commitments, values, and respects its employees.

Outright informing a superior or team lead that Bob is looking for another job put's him in a negative light, ultimately placing the blame on him and opening him up to immediate termination. As others have pointed out it is questionable behavior on your part

Approaching a superior or team lead and saying that we may lose Bob, an important part of the team, because the company didn't follow through on its promise of a transfer puts things in an entirely different light. It also opens up opportunities for the company to address the underlying issues and allows the company to resolve them if they chose to.

So what is your goal here?